Enzyme-based biosensors are devices in which an analyte-concentration-dependent biochemical reaction signal is converted into a measurable physical signal, such as an optical or electrical signal. Such biosensors are widely used in the detection of analytes in clinical, environmental, agricultural and biotechnological applications. Analytes that can be measured in clinical assays of fluids of the human body include, for example, glucose, lactate, cholesterol, bilirubin and amino acids. The detection of analytes in biological fluids, such as blood, is important in the diagnosis and the monitoring of many diseases.
Biosensors that detect analytes via electrical signals, such as current (amperometric biosensors) or charge (coulometric biosensors), are of special interest because electron transfer is involved in the biochemical reactions of many important bioanalytes. For example, the reaction of glucose with glucose oxidase involves electron transfer from glucose to the enzyme to produce gluconolactone and reduced enzyme. In an example of an amperometric glucose biosensor, glucose is oxidized by oxygen in the body fluid via a glucose oxidase-catalyzed reaction that generates gluconolactone and hydrogen peroxide, then the hydrogen peroxide is electrooxidized and correlated to the concentration of glucose in the body fluid.
Some biosensors are designed for implantation in a living animal body, such as a mammalian or a human body, merely by way of example. Typically, such biosensors have a three-electrode system provided with working electrodes which sensitively respond to species of interest, reference electrodes which control the potentials of working electrodes, and counter electrodes which pass the electrical currents generated on the working electrodes. Alternatively, the reference and counter electrodes can be combined as one electrode to form a two-electrode system. The working electrode is typically constructed of a sensing layer, which is in direct contact with the conductive material of the electrode, and a diffusion-limiting membrane layer on top of the sensing layer. The reference electrode is typically composed of Ag/AgCl, which is fabricated via screen printing or electroplating. However, the lifetime of a screen-printed Ag/AgCl reference electrode is typically limited in an in vivo amperometric sensor due to dissolution of the AgCl into the surrounding tissue.
As a result, the sensor's life as a whole is often limited by the amount of Ag/AgCl available on the sensor's reference electrode. Although increasing the level of Ag/AgCl loaded on the reference electrode can prolong the lifetime of the reference electrode, the small and compact size of an implantable biosensor prevents from doing so.
Therefore, there remains a need for providing a reference electrode having an extended lifetime that is suitable for long term use in an implantable biosensor. The present invention addresses this need.